Why doesn't accuracy always hold up?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by David P. Herne, Oct 27, 2003.

  1. David P. Herne

    David P. Herne Well-Known Member

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    Why doesn\'t accuracy always hold up?

    Gentlemen,
    I've always believed -- as I'm certain many of you have -- that if a given handload in a given rifle yields sub-0.5 or even sub-0.25 MOA accuracy at 100-200 yards, then one can extrapolate this accuracy to as far as the rifle/handload combination will reach, withstanding control of environmental/ meteorlogical variables, of course. But recently, I found two handloads using projectiles of different weight only (same manufacturer, same design, and same construction) that yield approximately the same level of accuracy at 100-200 yards, but which perform very differently at great distance (e.g. 1000 yards). Having just walked into the ranchhouse of a friend's spread here in east Texas where we drove last night to do some 1000 yard shooting with our .300 WSMs, I write you all this to tell you that a Lapua 167-grain Scenar load held up much better than the a Lapua 185-grain Scenar load. And this is the third time now that I've seen this in this rifle and with these same loads.

    Why does this happen?!?!?

    Dave
     
  2. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    Here are some of the reasons that the smaller bullet is doing a bit better at 1000 yards but, probably would not, if the range were further.
    keep in mind some bullets stabilize at different yardages and that could be happening here.

    1. The BC is close in both of these bullets.

    2. You are getting more velocity from the 167 gr and that may make the difference as per holding up a bit better.

    3. Try a comparable weight Sierra MK, JLK or Berger in that rifle once. [​IMG]

    later
    DC
     
  3. Varmint Hunter

    Varmint Hunter Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    DC,

    The lighter bullet, traveling at a higher velocity, would have a much higher rotational spin rate when exiting the muzzle. Could this be a factor in its ability to hold true to the initial moa noted at 200-300 yards?

    [​IMG]

    VH
     
  4. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    VH

    Hello
    That goes back to stabilization of the bullet.

    The two bullets are being fired from the same twist rifle so the rotaional spin is causing a slight stabilization point but, it shouldn't be to any major extent, especially at only 200 or 300 yards.

    If you were using a different twist then I would say that the rotaional spin and stabilazaton of the bullet would be much different and noticable at a bit shorter range..

    I don't think that is his problem here.

    I would try a comparable bullet with a high BC and even shoot further then 1000 yards to see the difference.

    Later
    DC [​IMG]

    [ 10-27-2003: Message edited by: Darryl Cassel ]
     
  5. David P. Herne

    David P. Herne Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    Gentlemen,
    Hmmm. So projectiles of different calibers and masses spin at different rates, independent of the barrel's twist? And as for the radical differences in accuracy, even when we begin from very much the same point, this seems to suggest that some bullets actually cork-screw, and not just spin, at different distances from the point they are fired?

    Dave
     
  6. Darryl Cassel

    Darryl Cassel Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    The added "velocity" of the lighter projective will change (increase) the spin rate causing an impact change at the target.
    Different weight bullets can also stabilize or destabilize at different yardages.

    Later
    DC
     
  7. Brent

    Brent Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    Dave,

    It just may be your 12 twist isn't stablizing the bullet well any longer. In marginally stabilized bullets this will become an issue as it approaches transonic.

    I'm fixin to try the 240 SMK in my 11 twist, ya think I'll nail the chrono as they go sideways? [​IMG]
     
  8. David P. Herne

    David P. Herne Well-Known Member

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    Re: Why doesn\'t accuracy always hold up?

    D.C.,
    Thank you for the feedback. Mind you, what you've said makes me think that the .408 CT might really do all those few who developed it say it will do at great distance, despite the fact that I've never seen anything in the way of respectable accuracy out of the things at distances as far as 600 yards save for a few fluke groups that measured sub-0.25 MOA.

    Brent,
    Damn, you've got a mind like a steel trap, amigo! I can't believe you remembered that I mentioned that my .300 WSM's tube has a 1:12 twist. By the way, I'm sorry I didn't get back to ya personally about the accuracy of this Lapua 185-grain load. Now you know!
    Incidentally, I've gotta e-mail you some pics of some of the extraordinary 20-round groups I've managed from a prone position (using a Harris bipod and Accu-Shot monopod) with this rifle and the subject Lapua 167-grain load. You'll be incredulous. The smith (Pete Pieper @ Precision Barrel Work) who did the rifle even wants to put up pics of it along with one of these targets at his website! Imagine that!

    Regards,
    Dave

    [ 10-28-2003: Message edited by: Houston Boy ]
     
  9. BountyHunter

    BountyHunter Writers Guild

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    Re: Why doesn\'t accuracy always hold up?

    From the obvious reasons everyone has listed here are some more for anyone shooting long range. Basically bullet uniformity is the key to exceptional LR grouping.

    1. Variations in the length of the bullets bearing surface within a given lot of bullets. This will effect the bc and often give vertical. I have seen variations as much as .020 in one box of 100 sierra bullets.

    2. Variance in the bullet metplats. Same as above.

    3. Variance in bullet weights in a given box or lot.

    There have been several very good LR smiths and shooters that long discovered and been preaching that the best way to accuracy is the "best quality and most uniform" bullets.

    Right now there is a move to Flat base LR match bullets in fast slow twist barrels. BIB has a great 187 that is shooting fantastic in 1-12 and up to 1-14 twist 30 cal barrels at 1000. Several 6.5 flat base bullets are coming out this year for 1-9 twists.

    BH
     
  10. Mysticplayer

    Mysticplayer Writers Guild

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    Re: Why doesn\'t accuracy always hold up?

    Houston Boy, since you are using a good quality bullet, I will leave bullet manf out of the discussion.

    What I would suggest is that the 1 in 12 twist of your barrel/muzz vel, is not spinning the bullet fast enough and the bullet is loosing stability at LR. Either increase muzzle vel or go to a faster twist and problem will correct itself. Remember that twist rate will vary even with the same numerical twist. Also, internal dimensions of the rifling will affect the true spin rate of the bullet.

    Are you using the same powder? Maybe the burn rate is not suited to the heavier bullet? A change in powder may help with vel variations and thus stringing. How are the vel spreads?

    Otherwise, the LR performance should be similar to groupings at ranges beyond 200yds (I like to test at 250yds) as long as wind is not a factor and the bullet stays supersonic.

    Jerry